Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Space Science

Committee Chair

Gary P Zank

Committee Member

Vladimir Florinski

Committee Member

Gary Webb

Committee Member

Jakobus Le Roux

Committee Member

David J McComas

Subject(s)

Heliosphere (Astrophysics), Shock waves., Plasma waves., Solar energetic particles., Ions.

Abstract

Shock waves occur frequently in the heliosphere and interstellar medium. They are a manifestation of the nonlinear character of plasma and can accelerate particles to very high energies. At a fundamental physics level, energetic particles determine the dissipative process governing the structure of shock waves. The observations made by many spacecraft demonstrate that the solar wind in the heliosphere is indeed mediated by nonthermal energetic particles such as solar energetic particles (SEPs) and pickup ions (PUIs). For instance, New Horizon observations showed that PUIs dominate the internal pressure in the outer heliosphere (McComas et al. 2017). Energetic PUIs are not equilibrated with the background thermal solar wind and have been observed to mediate collisionless shock waves in a variety of heliospheric environments. To understand the structure of shock waves in the heliosphere and the very local interstellar medium (VLISM), it is necessary to develop a theory and model to understand the effect of energetic particles on shock waves. This thesis investigates the effect of energetic PUIs on the structure of shock waves in different regions of the heliosphere and the VLISM. We then compare the results of our model with some observations. We use a PUI-mediated plasma model (Zank et al. 2014) and present a general theoretical model of parallel, perpendicular, and oblique shock mediation by energetic particles. We show that PUIs introduce dissipative effects through, for example, collisionless forms of the stress tensor, often expressed as a collisionless heat flux and viscosity. Such dissipative terms typically govern the structure of shock waves in the outer heliosphere. We show that the incorporation of both collisionless heat flux and viscosity associated with energetic particles can completely determine the structure of collisionless shocks. Voyager 2 observations of the heliospheric termination shock (HTS) showed that the HTS is not mediated by the thermal gas (Richardson et al. 2008). We use the PUI-mediated plasma model to study the structure of HTS and show that the thermal gas remains cold through the HTS and does not provide the dissipation needed to account for the deceleration of the supersonic solar wind whereas PUIs are the primary dissipation mechanism and gain most of the solar wind kinetic energy through the HTS. This thesis also studies different values of the HTS obliquity and finds that a parallel HTS heats PUIs more compared to the background thermal gas than it does at a perpendicular HTS. The low energy charged particle (LECP) instrument on Voyager 2 showed that the inner heliosheath (IHS; the region between the HTS and the heliopause) is dominated by the large energetic particle pressure and the plasma beta is very large (Decker et al. 2015). We show that interplanetary shocks propagating in this region are mediated by the energetic PUIs, and the PUIs represent the primary dissipation mechanism for perpendicular IHS shocks. IHS shocks enhance the IHS PUI temperature/pressure which leads to more charge exchange between IHS PUIs and interstellar neutral atoms. Therefore, our model shows that the presence of shock waves in IHS increases the production of energetic neutral atoms (ENAs), predicting an enhancement of the ENA flux that results in a better consistency with corresponding Interstellar Boundary Explorer (IBEX) observations. Voyager 1 and 2 crossed the heliopause in 2012 and 2018, respectively and are both making measurements of the VLISM. A shock wave was observed by Voyager 1 in the VLISM which was extremely broad and had properties very different than shocks in the heliosphere (Burlaga et al. 2013). We present a model to describe shocks in the VLISM and explain the difference between VLISM shocks and heliospheric shocks. We use the Chandrasekhar function to show that the electron and proton collisional mean free paths in the VLISM are relatively small. Therefore, the VLISM is collisional with respect to the thermal plasma, unlike the collisionless heliosphere. The thermal collisions introduce dissipation terms such as heat conduction and viscosity into the system. We show that the dominant collisional term in the VLISM is proton-proton collisions compared to other particle species collisions. VLISM PUIs do not introduce significant dissipation at interstellar shocks and VLISM shocks are thus not mediated by wave-particle interactions. We conclude that the VLISM shock structure is determined by thermal particle collisions and their broad thickness correspond to characteristic thermal heat conduction scale length.

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